1. Field of the Invention
The present invention relates to an apparatus and a method for positioning a mobile station in an LBS (location-based service) system, and more particularly to an apparatus and a method for calculating an E-OTD (enhanced observed time difference) and transmitting the E-OTD to a SMLC (serving mobile location center) using a mobile station.
2. Description of the Related Art
Positioning technologies used in an LBS include a network-based positioning method, a handset-based positioning method, and a hybrid positioning method which alleviate the defects of the first two methods.
The handset-based positioning method includes a TOA (time of arrival) method using a GPS (global positioning system), an E-OTD method wherein three or more BTSs (base transceiver stations) send radio waves to a mobile station and measure the returning time of the radio waves to provide positional information.
Recently, there is an increasing demand for a mobile station positioning service not only for personal purposes, but also to provide against various disasters and accidents.
FIG. 1 is a diagram illustrating the concept of the E-OTD method, among various positioning technologies for an LBS
An MS (mobile station) 100 receives a signal over a SCH (synchronization channel) from nearby BTSs (only one of them is indicated as 210 in the drawing) and measures the distance to the BTSs. An LMU (location measurement unit) 290 measures BCCHs (broadcast control channels) from nearby BTSs in a fixed location. An SMLC 270 calculates the location of the MS 100 based on the measurements transmitted from the MS 100 and the LMU 290.
For an MS to receive an SCH from a BTS, the MS needs to receive system information from a BCCH and obtain a BA (BCCH allocation) list.
As described in the GSM (global system for mobile communication) specification, an MS performs cell management using BA list information contained in system information 2 (2bis, 2ter), which is received in an idle mode, and in system information 5 (5bis, 5ter), which is received in a dedicated mode, among system information which is relayed through a BCCH in an RPLMN (registered public land mobile network) to which a serving cell, in which the MS is presently located, belongs. The BCCH is a channel which transmits system parameters, including information on operators, identifiers, locations of cells, movements of cells, frequencies, and the like.
The system information 2 is a nearby cell description used in an idle mode. The nearby cell refers to a cell the synchronization of which is obtained by an MS, among multiple cells belonging to a BA list. The nearby cell description refers to BCCH frequencies used by cells near a serving cell which is in a camp-on state. The “camp-on state” is a state where a calling party can wait for the answer from the called subscriber. Accordingly, this is called a BA list.
However, if locations of nearby BTSs, which are enumerated in the BA list, are difficult to calculate for geographical reasons, accuracy can deteriorate even when SCHs of the BTSs are received.
According to the 3GPP (3rd Generation Partnership Project) specification, the SMLC is adapted to load information on nearby BTSs, whose locations are convenient to calculate for geographical reasons, into assistance data and transmit it, so that the MS can decode the SCH.
Although the assistance data includes various information related to synchronization (for example, information on channel or timing of BTSs), it does not include the signal intensity of the SCH. This is because the signal intensity of the SCH sent by a BTS is changed in accordance with the location of an MS.
Referring to the GSM specification 3GPP TS 04.31 v8.8.0 (2002-02), the assistance data includes: number of BTSs (0-15); BCCH carrier (channel number); BSIC (base station identity code) of BTS; multi-frame offset from reference BTS; time slot scheme; approximate RTD (real time difference) between reference BTS; E-OTD (expected OTD); uncertainty of E-OTD value; and other information. The E-OTD refers to an OTD which is expected between the reference BTS.
FIG. 2 is a flow chart illustrating a method wherein a conventional mobile station measures its location using assistance data.
In step 2a, a location-measuring request is received through an RRLP. The location-measuring request is created by an SMLC and is transmitted by a BTS. A response time value, which is defined as 2 seconds, 4 seconds, 8 seconds, and the like, according to the specification, is included in the location-measuring request. The mobile station is supposed to send a location-measuring response within the response time.
In step 2b, assistance data through the RRLP is received and information related to the timing of a BTS is detected from the assistance data. The timing information of a BTS includes multi-frame offset, an E-OTD (expected OTD), and the like.
In step 2c, a BTS, the SCH of which is to be measured, is determined based on the response time value. If a BTS can measure an SCH and send a response within a limited time, that BTS is chosen. This can be done by detecting multi-frame offset, which tells the location of the SCH.
In step 2d, the SCH of the chosen BTS is received.
In steps 2e and 2f, the E-OTD value of each of the BTSs relative to a reference BTS is calculated based on information included in the received SCH, and a response, including the calculated value, is transmitted to the SMLC through the RRLP.
In step 2g, if assistance data is not received during step 2b, a BTS which is enumerated in a BA list is chosen and step 2d is performed.
Meanwhile, frequency numbers of the BCCHs from respective BTSs are defined in the assistance data and a few of them are chosen to receive the SCH. This choice is made by service providers according to their own algorithms. If the choice is inappropriate, the accuracy of the calculated value of the E-OTD may be degraded. A specific example could be given as follows:
Assume that seven out of fifteen BTSs are chosen. One of the chosen BTSs has very weak signal intensity, while the remaining eight BTSs, which are not chosen, each have a signal intensity which is relatively stronger than the one BTS. Under this assumption, when an MS fails to receive an SCH from a BTS due to weak signal intensity, it cannot receive an SCH from another BTS because the time has expired. Accordingly, the choice of a BTS can play a role in determining the accuracy of the E-OTD value.
Furthermore, there is no method for obtaining a receipt gain for BTSs which are not defined in the BA list, although their BCH frequency numbers are defined in the assistance data. In that case, receipt gains which have been measured periodically should be used. This decreases the possibility of receiving SCHs and degrades the accuracy in calculating locations.